There is a biosensor for measuring a specific measurement target substance in a specimen liquid, which measures an electric current value obtained by a reaction between glucose in blood and a reagent such as glucose oxidase, potassium ferricyanide, or the like supported in the sensor, thereby to obtain a blood sugar level.
FIG. 3 is an exploded diagram illustrating a process of manufacturing a conventional biosensor for measuring a blood sugar level.
A pair of conductive lead parts 2 and 3 stretching from a working electrode and a counter electrode to measuring terminals 2a and 3a is formed on a film insulating substrate 1 comprising polyethylene terephthalate or the like by screen printing or the like employing silver paste. End parts 2b and 3b of the conductive lead parts 2 and 3 are shaped to follow roughly a working electrode and a counter electrode to be formed afterwards. That is, the end part 2b of the conductive lead 2 is formed into a rectangle shape and the end part 3b of the conductive lead 3 is formed into a shape surrounding the rectangular shape. Then, a working electrode 4 and a counter electrode 5 of prescribed shapes are formed so as to overlap the respective end parts 2b and 3b, employing carbon paste.
Next, an insulating paste is overprinted on the insulating substrate 1 so as to expose the working electrode 4, the counter electrode 5, and the connection terminals 2a and 3a, thereby forming an insulating layer 6. A reaction layer 7, which includes carboxymethyl cellulose as a hydrophilic polymer, glucose oxidase as an enzyme, and potassium ferricyanide as an electron acceptor, is formed on the exposed working electrode 4 and counter electrode 5 so as to bridge these electrodes 4 and 5.
Thereafter, a cover, onto the reverse side of which a spacer 8 with a spindly specimen supply groove 10 having an opening part at its end formed is attached, is adhered so that an end part of the specimen supply groove 10 is located on the reaction layer 7, so as to cover the reaction layer 7 with the connection terminals 2a and 3a being left, as shown in FIG. 4. Numeral 11 denotes an air vent formed at the end part of the specimen supply groove 10.
When the sensor constructed as described above is connected to a measuring device and a blood sample to be measured is brought into contact with the opening of the specimen supply groove 10, a prescribed amount of sample is introduced into the reaction layer 7 through the specimen supply groove 10 by a capillary phenomenon, and a prescribed reaction between the glucose in the blood and the glucose oxidase as well as potassium ferricyanide supported in the sensor is developed. Then, an electric current value accompanying the reaction is read on the measuring device side through the connection terminals 2a and 3a, and the content of the glucose as the measurement target substance is measured from the electric current value.
However, in the case of the above-described conventional biosensor, the working electrode 4 and the counter electrode 5 are formed by overprinting carbon electrodes having almost the same shapes as the end parts 2b and 3b of the conductive lead parts which are formed of silver, by screen printing. Therefore, a pin hole or crack may be generated in the carbon electrode due to the printing state of carbon, the drying temperature, the attachment pressure of a top cover, or the like, whereby the conductive lead parts beneath the working electrode 4 and the counter electrode 5 are exposed to the surface to come into contact with the potassium ferricyanide which is the electron acceptor in the reaction layer 7, resulting in an increase in blank value and degradation of CV value (complete blood accuracy).
Further, regarding the working electrode, an increase in blank value and degradation of CV value occur due to an oxidation current of silver, resulting in degradation of sensor accuracy. Since the above-described problems have larger influences in a high-humidity environment, preservation in a dry condition is indispensable.